Melt Forms

MORPHOLOGICAL CLASSIFICATION ADDITIONAL INFORMATION ON PHYSICAL PROCESS AND STRENGTH

BASIC CLASSIFICATION SUBCLASS SHAPE CODE PLACE OF FORMATION PHYSICAL PROCESS DEPENDENCE ON MOST IMPORTANT PARAMETERS COMMON EFFECT ON STRENGTH

MELT FORMS

h

MF

Clustered rounded grains 

Rounded polycrystals

M

Clustered rounded crystals held by large ice-to-ice bonds; water in internal veins among three crystals or two grain boundaries

Individual crystals are frozen into a solid polycrystalline particle, either wet or refrozen

Slush N

Separated rounded particles completely immersed in water

Meltfreeze crust Oh

Crust of recognizable melt-freeze polycrystals MFcr At the surface Crust of melt-freeze polycrystals from a surface layer of wet snow that refroze after having been wetted by melt or rainfall; found either wet or refrozen Particle size and density increases with number of melt-freeze cycles Strength increases with number of melt-freeze cycles

MFcl At the surface or within the snowpack; wet snow

MFpc At the surface or within the snowpack Wet snow at low water content (pendular regime), i.e., holding free liquid water; clusters form to minimize surface free energy Melt-freeze cycles form polycrystals when water in veins freezes; either wet at low water content (pendular regime) or refrozen Meltwater can drain; too much water leads to MFsl; first freezing leads to MFpc

Particle size increases with number of melt-freeze cycles; radiation penetration may restore MFcl; excess water leads to MFsl

MFsl Water saturated, soaked snow; found within the snowpack, on land or ice surfaces, but also as a viscous floating mass in water after heavy snowfall. Wet snow at high liquid water content (funicular regime); poorly bonded, fully rounded single crystals–and polycrystals–form as ice and water are in thermodynamic equilibrium Water drainage blocked by capillary barrier, impermeable layer or ground; high energy input to the snowpack by solar radiation, high air temperature or water input (rain) Ice-to-ice bonds give strength

High strength in the frozen state; lower strength in the wet state; strength increases with number of melt-freeze cycles Little strength due to decaying bonds

Melt Form Notes: Melt-freeze crusts MFcr form at the surface as layers at most a few centimeters thick, usually on top of a subfreezing snowpack. Rounded polycrystals MFpc will rather form within the snowpack. MFcr usually contain more refrozen water than MFpc and will not return to MFcl. Both MFcr and MFpc may contain a recognizable minority of other shapes, particularly large kinetic growth form FC and DH. See the guidelines (Appendix C) for examples on the use of the MFcr symbol.